In patients with Alzheimer's disease, a higher burden of Chlamydia pneumoniae bacteria was found in the retina and brain compared to the control group.[1] This burden increased with the presence of the APOE4 gene, the advanced stage of the disease and the degree of cognitive deficits.[1] The bacterium Chlamydia pneumoniae, originally a respiratory pathogen, has been repeatedly identified in the brains of Alzheimer's patients, often in association with amyloid plaques and neurofibrillary tangles.[1][2] It can enter the brain through the olfactory pathway, immune cells, or systemic inflammation and infect astrocytes and microglia, leading to chronic neuroinflammation and neuronal damage.[1][2] Studies in models have shown that the infection triggers the formation of amyloid plaques and neuroinflammation similar to Alzheimer's disease.[2][4] One cohort study in Taiwan confirmed a higher risk of Alzheimer's dementia in pneumonia caused by Chlamydia pneumoniae, while treatment with macrolides or fluoroquinolones (at least 15 days) reduced this risk.[3] The presence of the bacteria in the brain correlates with higher antibody levels, indicating chronic infection.[1]